This invention relates to a read-out method and control circuit for a light-sensing and light-emitting diode array that operates in both a read mode for scanning documents and a write mode for printing.
Facsimile machines and other devices that must both scan and print can employ an electrophotographic printing technique in which a latent image is created by illuminating the surface of a photosensitive drum. The latent image is developed by applying toner, then transferred to paper. The light source that illuminates the photosensitive drum may be an array of light-emitting diodes (LEDs). The array can also be biased so as to function as an array of light-sensing diodes. The same LED array can thus provide both the light needed to create printable images in write mode, and the light-sensing capability needed for scanning documents in read mode.
In read mode, the array can be read by detecting charges stored in the capacitances of the pn junctions of the LEDs. This is conventionally done by charging the LEDs in turn through a resistor and sensing the peak voltage produced by the charging current for each diode. This conventional method produces an output waveform that swings from ground level up to a peak voltage and then back to ground again as each LED is read.
One disadvantage of this conventional method of reading an LED array is that it produces a weak, but rapidly-varying signal. To convert this signal to suitable input for an analog-to-digital converter, the signal must be amplified by several high-speed operational-amplifier stages. Moreover, since it is the peak values of the output signal that are significant, a peak-detecting circuit is necessary. The control circuitry thus requires a large number of expensive peripheral circuits.
Another disadvantage of tile conventional method is that, since the LED array usually consists of a plurality of monolithic semiconductor chips, with a corresponding plurality of monolithic integrated control circuits mounted on a printed wiring board, the weak output signals from the control circuits are easily distorted as they travel through printed wiring traces on the board to a common output terminal. The distortion can be produced by the stray inductance and capacitance of the wiring traces, and by electromagnetic noise.